Production of synthetic patina

ABSTRACT

A DECORATIVE SYNTHETIC PATINA IS PRODUCED ON A CUPREOUS SURFACE BY COATING THE SURFACE WITH AN AQUEOUS ACID SOLUTION CONTAINING SALTS IN ABOUT THE FOLLOWING WEIGHT PERCENT, DRY BASIS, 10-20% IRON SULFATE, 13-21% SULFAMIC ACID, AND THE BALANCE AMMONIUM SULFATE, THE REQUIRED STRENGTH OF THE BATH BEING A FUNCTION OF THE WITHDRAWAL RATE OF THE CUPREOUS SURFACE. THE SALTS THUS APPLIED ARE DRIED FORMING A REACTANT COATING ON THE CUPREOUS SURFACE. THESE SALTS ARE THEN REACTED WITH THE CUPREOUS SURFACE UNDER CONTROLLED TEMPERATURE AND HUMIDITY TO FORM AN ADHERENT CORROSION PRODUCT. FINALLY, THE SOLUBLE PARTS OF THE COATING ARE DISSOLVED OUT, DEVELOPING THE COLOR OF NATURAL PATINA ON THE SURFACE. THE COLOR PRODUCED MAY BE CONTROLLED CLOSELY BY VARYING THE TOTAL AMOUNT, AND THE FERRIC-FERROUS RATIO, OF THE IRON SALTS INCLUDED IN THE AQUEOUS ACIDIC SOLUTION. AN ARTICLE COLORED BY THE PROCESS HAS A NATURAL PATINA APPEARANCE HAVING A CORROSION PRODUCT WITH ABOUT THE FOLLOWING PERCENTAGES BY WEIGHT: 47-54% COPPER, 1723% SULFATE, APPROXIMATELY 1% SULFAMATE, 0.7-2% IRON, WATER LESS THAN 3% AND THE REMAINDER HYDROXIDE.

Aprll 3, 1973 R. B. JONES PRODUCTION OF SYNTHETIC PATINA Filed March 15 1971 RONALD B. L. JONES I INVENTOR MUM. WW

ATTORNEYS United States Patent 3,725,138 PRODUCTION OF SYNTHETIC PATINA Ronald B. L. Jones, Watertown, Conn., assignor to Anaconda American Brass Company, Waterbury, Conn. Continuation-impart of abandoned application Ser. No.

762,925, Sept. 26, 1968. This application Mar. 15, 1971,

Ser. No. 124,480

Int. Cl. C23f 7/00 U.S. Cl. 148-614 15 Claims ABSTRACT OF THE DISCLOSURE A decorative synthetic patina is produced on a cupreous surface by coating the surface with an aqueous acid solution containing salts in about the following weight percent, dry basis, 10-20% iron sulfate, 13-21% sulfamic acid, and the balance ammonium sulfate, the required strength of the bath being a function of the withdrawal rate of the cupreous surface. The salts thus applied are dried forming a reactant coating on the cupreous surface.

- These salts are then reacted with the cupreous surface RELATED APPLICATIONS This application is a continuation-in-part of U.S. application Ser. No. 762,925 of Ronald B. L. Jones, filed Sept. 26, 1968, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method of forming a synthetic patina having the color of natural patina on a cupreous surface and to articles formed thereby.

Heretofore, various salt solutions have been used to develop artificial patina colorations on copper but the color realized by the prior art generally failed to duplicate the color of the natural patina. The color varied so greatly depending upon the composition of the copper surface and the chemicals used as to be commercially unacceptable. Often the film resulting from the mixture of the chemicals with the copper required additional Work such as hand toning or repetitious treatment with a variety of chemicals in order to obtain the desired adherence and color.

The use of an electrolyte consisting of salts and a hydroxide in attempting to anodize the copper has also been tried but the copper still requires aging to develop the patina fully. Numerous other attempts have been made by the prior art but in general they have either failed to develop the color of the natural patina or they have not been commercially practicable.

SUMMARY OF THE INVENTION The invention relates to a method of imparting a decorative synthetic patina to a cupreous surface in which the surface is coated with an aqueous acidic solution containing in percentages on a dry basis from balance ammonium sulfate, 13-21% by weight sulfamic acid and 10-20% by weight iron sulfate, with the required strength of the bath being a function of the Withdrawal rate of the cupreous surface. A reactant film is then formed on the surface as the coating is dried. The film is then reacted with the cupreous surface to form a corrosion reaction product which is firmly adhered to the surface with a major portion of the reaction product being insoluble. Thereafter the soluble portion of the corrosion reaction product is dissolved and the color of natural patina is developed in the reaction product during dissolution. The color is controlled by varying the total amount, and the ferric-ferrous ratio, of the iron salts included in the aqueous acidic solution.

While not limited thereto, the method lends itself to a continuous process and it is preferred to use a solution with a pH of from about 1.2-2 having the following composition in percentage by weight, dry basis: balance ammonium sulfate, 13-21% sulfamic acid, 7-l3% ferrous surface, with about of this mixture being insoluble. These combined salts are addedto a bath in a range of about 150 to 400 grams of dissolved salts per liter. The copper strip is dipped into the bath, withdrawn at a uniform'rate, dried, preheated to a temperature of 65- 100 C. and subjected to a controlled reactant environment having a temperature of from 30-60 C. and a relative humidity of -96% to control the reaction between the film and the copper surface. A dark mixture of salts and corrosion products is formed on the surface, with about 75 of this mixture being insoluble. The strip is then dipped in a Water bath where most of the soluble parts of the reaction mixture are removed, leaving insoluble corrosion products which when dry give to the copper surface a color almost identical to that of natural patina.

Articles colored by this method are coated with a patina having the following percentages by Weight: 47- 54% copper, 17-23% sulfate, approximately 1% sulfamate, .7-2% iron, less than 3%. water, and the remainder hydroxide.

It will be recognized that since patina is often desired on only one surface of the copper strip, reaction on the other surface can be prevented by removing therefrom, by squeegee or other means, the reactive salt solution picked up during immersion.

BRIEF DESCRIPTION OF THE DRAWINGS A schematic drawing of a continuous coil process for patinating copper strip is shown.

DESCRIPTION OF THE PREFERRED EMBODIMENT A coil of copper strip having a 0.020 gauge is stripped from roll 10 at a rate of 3.4 feet per minute and transferred to a vapor degreasing station 11 and then to a chromic acid solution tank 12 where it is immersed for pickling. The strip is continuously fed to a spray rinse 13 where it is sprayed on both sides with water and then to a drying station 14 where it is dried. The strip is then continuously passed to a tank 15 which contains an aqueous solution having 200 g. of dissolved salts per liter; the salts having the following composition in percent by weight:

As shown the copper strip was passed around a dip roller 16 that is immersed in the acid solution and is withdrawn vertically from the tank at a rate of 3.4 f.p.m. to emerge coated with a thin film. As the aqueous coating was withdrawn vertically from the bath it is dried between heaters 17 preferably within about eight seconds from leaving the tank so as to guarantee formation of fine uniformly distributed salt crystals and to prevent formation of large or long crystals. It was found that the salts adhered to the cupreous surface ranged from 2.8 to 3.8 mg./in. /side.

When heaters 17 are used they dry the coating and preheat the strip to a temperature above that at which condensation would occur on introduction to the controlled environment. Preheat temperatures should fall within a range of about 65 C. to about 100 C. with 80 or 85 C. being preferred. The preheated strip is then passed into the environmental station 18 where the temperature is retained through its length at about 50 C. and a relative humidity of about 89% with the preferred ranges being between about 45 C. and 60 C. and a relative humidity of about 80% to about 96% for about one or two hours. These conditions are held substantially constant throughout the length of the station.

Alternatively, the coating may be dried without use of heaters by controlling the temperature and relative humidity in a portion of environmental station 18 to cause a sufiicient amount of drying to take place in such portion of the station before the coated strip passes into and through the remaining portion of the station where further treatment takes place, the combined drying and further treatment times being about one to two hours. When the station is used to accomplish drying, the strip enters one end of the station and exits the other end of the station unlike the doubling-back arrangement shown in the drawings. To accomplish both drying and reacting in the station the entrance temperature is in the range of 60 C. to about 70 C.; entrance relative humidity is in the range of about 40% to about 60% with about 40% preferred; exit temperature is in the range of about 45 C. to about 60 C. with 50 C. preferred and finally the exit relative humidity is in the range of about 80% to about 96% with about 90% being preferred. The conditions along the length of the station will vary substantially proportionally between the entrance and exit conditions: for example, the preferable relative humidity in the middle of the station would be about 65%, midway betwen 40% and 90%. The conditions in the entrance section of the station provide a low humidity drying atmosphere to accomplish sufficient formation of crystals on the strip surface. The station has sufiicient length so that the strip is, subsequent to the drying step, subjected to a reaction atmosphere at the exit section of the station having a humidity at or above 80% with a temperature at or above 45 C. for a suflicient length of time to form the adhered corrosion product.

It is the objective during exposure in the reacting atmosphere to effect a corrosion reaction in which the surface salts with the aid of moisture and oxygen from the reacting atmosphere convert a small amount of the underlying copper to basic copper sulfate and other products, thereby forming a patina. The temperature, humidity and time relationships are significant; if the humidity is too high, a mottled appearance will occur; if the humidity is too low, there will be inadequate adherence; if too little time is used, there will be insufficient adherence, and too dark a reaction product will result if the strip is left in the station too long. The time may vary but the reaction should at least be carried on long enough to form an adhered corrosion product of which a major portion is insolubilized.

The corrosion product formed after reaction in the environmental station is a dark product in which about 75% is insoluble in water, with the remaining 25% being soluble. Upon removal from the environmental station,

the strip is immersed in a tank 19 containing water at room temperature for about fifteen minutes during which time about 8090% of the soluble salts are removed from the corrosion product and at the same time considerable lightening of the color of the corrosion product to that of natural patina occurs. 'In general, about 1530% of the corrosion product which includes some excess soluble reactant salts will be removed during the leaching operation. Upon removal from immersion tank 19, the strip is passed through heaters 20 and dried to leave a residual corrosion product of about 5 mg./in. It has been found that the range of weight of coating will be from 3-7 mg./in. The color was controlled by varying the total amount, and the ferric-ferrous ratio, of the iron salts included in the aqueous acid solution within the ranges given above.

An analysis of the corrosion products having the natural patina color has given the following composition range:

Percent Copper 47-54 Sulfate 17-23 sulfamate, approximately 1 Iron .7-2 Water 3 Hydroxide Remainder The method for sulfamate determination was as follows:

1) Dissolve off about 5 g. of artificial patina from copper pieces by submerging in 20 ml. 1:1 HCl. Dilute to about 100 ml. Filter and dilute to 400 ml. Take 100 ml. of this solution to do the test. Dilute the 100 ml. aliquot to about 400 ml. using a 600 ml. beaker.

(2) Add some hydroxylamine hydrochloride to reduce Fe' to Fe.

(3) Heat to boiling. Remove from the heat.

(4) Dilute 20 ml. of 5% BaCl .2H O to 50 ml., heat nearly to boiling and add the solution, a dropper full at a time with constant stirring to the sulfate solution.

(5) Allow the precipitate to settle overnight.

(6) Filler the precipitate using #429 cm. Whatman filter paper saving the filtrate. Wash the precipitate and paper about eight times with water. Test the last few milliliters of washings with a drop of silver nitrate for the presence of chloride, this ion being indicative of incomplete washing, continue washing until no chloride ions are present. Place the filter paper containing the precipitate into a weighed porcelain crucible previously ignited to redness and cooled in a desiccator. Dry the paper and ash it, can heat the precipitate for 10 to 15 minutes at moderate redness. Place the crucible in a desiccator to cool to room temperature. Weigh and repeat the ignition with 10 minute intervals of heating until constant weight (0.2 mg.) is obtained.

(7) To the filtrate obtained in step 6, add 0.5 g. sodium nitrite (NaNO Heat to boiling to oxidize sulfamate to sulfate. Continue heating until the volume is about 100 ml.

(8) Hold the precipitate overnight.

(9) Filter as described in step 6 above.

(10) The weight of the ash as prepared in step 9 is considered to be the barium sulfate equivalent to sulfamate ion.

This analytical procedure was followed to determine the presence of sulfamate in natural patina and in synthetic patina produced according to this invention. Scrapings were taken from natural patina and from synthetic patina of the invention and based on this analytical procedure, the confidence ranges for sulfamate are as follows: natural patina 0.120.15% SO NH synthetic patina 1.01.1% SO3NH Thus I have found that the natural patina will have a nominal amount of sulfamate and not more than about 0.13% but the synthetic patina has a nominal amount of sulfamate of approximately 1%, almost one order of magnitude greater than that contained in the natural patina.

I claim:

1. A method of imparting a decorative synthetic patina finish to a cupreous surface comprising:

(a) applying to a cupreous surface an aqueous acidic solution containing percentage ranges by weight, dry basis: balance ammonium sulfate, 13-21% sulfamic acid and -20% iron sulfate, and drying the solution to form a film of reactant salts on the surface,

(b) reacting the film with the cupreous surface by placing the film-bearing cupreous surface in a reacting atmosphere having a humidity of at least 80% and a temperature of at least 30 C. for a sufiicient period of time to form an adhered corrosion product of which a major portion is insolubilized, and

(c) dissolving in an aqueous solution soluble parts out of the reaction product thereby developing in the reaction product the color of natural patina.

2. The method of claim 1 in which such drying is accomplished by heating the coated strip prior to its introduction in such reacting atmosphere.

3. The method of claim 1 in which drying is accomplished by passing the coated strip through a drying atmosphere having a relative humidity below about 60% and a temperature above about 60 C. and thereafter passing the strip through said reacting atmosphere.

4. The method of claim 2 in which the reacting atmosphere has a humidity in the range of about 80% to about 96% and a temperature range of about 45 C. to about 60 C.

5. The method of claim 3 in which the drying atmosphere has humidity in the range of about 40% to about 60% and a temperature in the range of about 60 C. to about 70 C.

6. A method according to claim 2 comprising drying the surface by heating the coated cupreous surface to a temperature of from about 65 to 100 C. and holding in this temperature range until the preheated coated surface is placed into a controlled environment having a temperature of from 45 to 55 C. and a relative humidity of 80 to 96% and holding the coated surface in this environment for about one hour to form an adhered corrosion product.

7. A method according to claim 1 wherein the reaction product having the color of natural patina contains the following percentages by weight:

Percent copper 47-54 sulfate 17-23 sulfamate, approximately 1 iron 0.7-2 Water 3 hydroxide 1 Remainder pH of from about 1.2 to 2 and containing from about -400 g./l. of the following solute in percentages by weight, dry basis:

Percent ammonium sulfate Balance sulfamic acid 13-21 ferrous sulfate heptahydrate 7-13 ferric sulfate hexahydrate 3.5-7 wetting agent Trace 10. A method according to claim 1 wherein the aqueous sulfamic acid solution had a pH of 1.3-1.7 and the following composition in percent by weight, dry basis:

Percent ammonium sulfate Balance sulfamic acid 16.8-17.8

ferrous sulfate heptahydrate 9.4-10.0 ferric sulfate hexahydrate 4.9-5.3 wetting agent Trace 11. A method according to claim 10 wherein the cupreous surface is immersed in the aqueous sulfamic acid acidic solution of such concentration, and withdrawn at such a rate, that approximately 2.8-3.8 mg./in. of dried reactive salts are retained on the cupreous surface.

12. A method according to claim 10 wherein the cupreous surface is immersed in the aqueous sulfamic acid acidic solution and withdrawn at a rate of approximately 3.4 f.p.m. to adhere to the copper surface 2.8 to 3.8 mg./in. /side of dried reactive salts.

13. A method according to claim 1 wherein the color is controlled by varying the total amount, and the ferricferrous ratio, of the iron salts included in the aqueous acidic solution.

14. An article having a cupreous surface on which is adhered a corrosion product with a recorative natural patina color, the corrosion product having a composition in percentages by weight comprising:

Percent copper 47-54 sulfate 17-23 sulfamate, approximately 1 iron 0.7-2 water 3 hydroxide t.. Remainder 15. An article according to claim 14 wherein from 3-7 mg./in. of said reaction product is on the cupreo-us surface.

References Cited UNITED STATES PATENTS 3,473,970 10/ 1969 Robey l48-6.24X 3,497,401 2/ 1970 Hanson et al. 1486.24

RALPH S. KENDALL, Primary Examiner C. B. WESTON, Assistant Examiner US. Cl. X.R. 148-624 

